5706: handle message attachments

bindings/java/ivy.xml

bindings/java/nbproject/project.xml

bindings/java/src/org/sleuthkit/datamodel/Blackboard.java

...java/src/org/sleuthkit/datamodel/BlackboardAttribute.java

bindings/java/src/org/sleuthkit/datamodel/Bundle.properties

bindings/java/src/org/sleuthkit/datamodel/SleuthkitCase.java

...tamodel/blackboardutils/CommunicationArtifactsHelper.java

...g/sleuthkit/datamodel/blackboardutils/FileAttachment.java

...euthkit/datamodel/blackboardutils/MessageAttachments.java

...rg/sleuthkit/datamodel/blackboardutils/URLAttachment.java